Bowed rotor start mitigation in a gas turbine engine using aircraft-derived parameters

ABSTRACT

A bowed rotor start mitigation system for a gas turbine engine of an aircraft is provided. The bowed rotor start mitigation system includes a motoring system and a controller coupled to the motoring system and an aircraft communication bus. The controller is configured to determine at least one inferred engine operating thermal parameter from at least one aircraft-based parameter received on the aircraft communication bus. The motoring system is controlled to drive rotation of a starting spool of the gas turbine engine below an engine idle speed based on determining that the at least one inferred engine operating thermal parameter is within a preselected threshold.

BACKGROUND

This disclosure relates to gas turbine engines, and more particularly to an apparatus, system and method for mitigating a bowed rotor start condition in a gas turbine engine using aircraft-derived parameters.

Gas turbine engines are used in numerous applications one of which is for providing thrust to an airplane. When the gas turbine engine of an airplane has been shut off for example, after an airplane has landed at an airport, the engine is hot and due to heat rise, the upper portions of the engine will be hotter than lower portions of the engine. When this occurs thermal expansion may cause deflection of components of the engine which may result in a “bowed rotor” condition. If a gas turbine engine is in such a “bowed rotor” condition it is undesirable to restart or start the engine.

Accordingly, it is desirable to provide a method and/or apparatus for detecting and preventing a “bowed rotor” condition.

BRIEF DESCRIPTION

In an embodiment, a bowed rotor start mitigation system for a gas turbine engine of an aircraft is provided. The bowed rotor start mitigation system includes a motoring system and a controller coupled to the motoring system and an aircraft communication bus. The controller is configured to determine at least one inferred engine operating thermal parameter from at least one aircraft-based parameter received on the aircraft communication bus. The motoring system is controlled to drive rotation of a starting spool of the gas turbine engine below an engine idle speed based on determining that the at least one inferred engine operating thermal parameter is within a preselected threshold.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the at least one inferred engine operating thermal parameter is selected from data describing a history of the aircraft before an engine shutdown.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where a time since the engine shutdown is used to modify a time of rotation of the starting spool by the motoring system.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the controller transmits a notification on the aircraft communication bus that the gas turbine engine is being conditioned for starting.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the controller is configured to abort rotation by the motoring system when a safety condition is detected.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the motoring system is configured to apply a braking torque to the starting spool to abort rotation of the starting spool.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include a rotation sensor for detecting rotary motion of the starting spool, and the controller is further coupled to the rotation sensor.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the controller is configured to remove power from the motoring system based on detecting rotary motion of the starting spool above a target speed.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where a vibration monitor is used to set a maintenance flag.

In addition to one or more of the features described above, or as an alternative to any of the foregoing embodiments, further embodiments may include where the at least one inferred engine operating thermal parameter includes an engine temperature value, and the at least one aircraft-based parameter received on the aircraft communication bus includes a throttle lever angle profile.

According to an embodiment, a method of bowed rotor start mitigation for a gas turbine engine of an aircraft is provided. The method includes determining at least one inferred engine operating thermal parameter from at least one aircraft-based parameter received on an aircraft communication bus. The method also includes controlling a motoring system to drive rotation of a starting spool of the gas turbine engine below an engine idle speed based on determining that the at least one inferred engine operating thermal parameter is within a preselected threshold.

A technical effect of the apparatus, systems and methods is achieved by using a start sequence for a gas turbine engine as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the present disclosure is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the present disclosure are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a cross-sectional view of a gas turbine engine;

FIG. 2 is a schematic illustration of a starting system for a gas turbine engine in accordance with an embodiment of the disclosure;

FIG. 3 is a schematic illustration of a starting system for a gas turbine engine in accordance with another embodiment of the disclosure;

FIG. 4 is a block diagram of a system for bowed rotor start mitigation in accordance with an embodiment of the disclosure;

FIG. 5 is a flow chart illustrating a method of bowed rotor start mitigation of a gas turbine engine in accordance with an embodiment of the disclosure;

FIG. 6 is a graph illustrating a bowed rotor risk score with respect to time in accordance with an embodiment of the disclosure;

FIG. 7 is a graph illustrating a normal or cooled engine start versus a modified engine start in accordance with an embodiment of the disclosure;

FIG. 8 is a graph illustrating examples of various vibration level profiles of an engine in accordance with an embodiment of the disclosure;

FIG. 9 is a schematic illustration of a high spool gas path with a straddle-mounted spool in accordance with an embodiment of the disclosure;

FIG. 10 is a schematic illustration of a high spool gas path with an overhung spool in accordance with an embodiment of the disclosure;

FIG. 11 is a graph illustrating commanded starter valve opening with respect to time in accordance with an embodiment of the disclosure; and

FIG. 12 is a graph illustrating a target rotor speed profile of a dry motoring profile and an actual rotor speed versus time in accordance with an embodiment of the disclosure.

DETAILED DESCRIPTION

Various embodiments of the present disclosure are related to a bowed rotor start mitigation system in a gas turbine engine. Embodiments can include a mission risk factor model used to estimate heat stored in an engine core at shutdown and identify a risk of a bowed rotor in the gas turbine engine based on one or more aircraft parameters. Information from the mission risk factor model can be used by a bowed rotor start risk model to calculate a bowed rotor risk parameter. As used herein the term “model” may be referred to in one non-limiting manner as a process for representing real world values, measurements or conditions through the use of a computer program or algorithm. The bowed rotor risk parameter may be used to take a control action to mitigate the risk of starting the gas turbine engine with a bowed rotor. The control action can include performing dry motoring as further described herein.

During dry motoring, a starter valve can be actively adjusted to deliver air pressure from an air supply to an engine starting system that controls starting rotor speed. Dry motoring may be performed by running an engine starting system at a lower speed with a longer duration than typically used for engine starting while dynamically adjusting the starter valve to maintain the rotor speed and/or follow a dry motoring profile. Some embodiments increase the rotor speed of the starting spool to approach a critical rotor speed gradually and as thermal distortion is decreased they then accelerate beyond the critical rotor speed to complete the engine starting process. The critical rotor speed refers to a major resonance speed where, if the temperatures are unhomogenized, the combination of a bowed rotor and similarly bowed casing and the resonance would lead to high amplitude oscillation in the rotor and high rubbing of blade tips on one side of the rotor, especially in the high pressure compressor if the rotor is straddle-mounted.

In some embodiments, a targeted rotor speed profile of the dry motoring profile can be adjusted as dry motoring is performed. As one example, if excessive vibration is detected as the rotor speed rises and approaches but remains well below the critical rotor speed, then the rate of rotor speed increases scheduled in the dry motoring profile can be reduced (i.e., a shallower slope) to extend the dry motoring time. Similarly, if vibration levels are observed below an expected minimum vibration level as the rotor speed increases, the dry motoring profile can be adjusted to a higher rate of rotor speed increases to reduce the dry motoring time.

A full authority digital engine control (FADEC) system or other system may send a message to the cockpit to extend time at idle to cool down the rotor prior to shut down. If the engine is in a ground test or in a test stand, a message can be sent to the test stand or cockpit based on the control-calculated risk of a bowed rotor. A test stand crew can be alerted regarding a requirement to bring the starting spool of the engine to a speed below the known resonance speed of the rotor in order to homogenize the temperature of the rotor and the casings about the rotor which also are distorted by temperature non-uniformity.

Monitoring of vibration signatures during the engine starting sequence can also or separately be used to assess the risk that a bowed rotor start has occurred due to some system malfunction and then direct maintenance, for instance, in the case of suspected outer air seal rub especially in the high compressor. Vibration data for the engine can also be monitored after bowed rotor mitigation is performed during an engine start sequence to confirm success of bowed rotor mitigation. If bowed rotor mitigation is unsuccessful or determined to be incomplete by the FADEC, resulting metrics (e.g., time, date, global positioning satellite (GPS) coordinates, vibration level vs. time, etc.) of the attempted bowed rotor mitigation can be recorded and/or transmitted to direct maintenance

According to various embodiments, there are a number of options available to mitigate a bowed rotor start depending on a present operating state of the gas turbine engine, instrumentation, and monitoring systems implemented. For example, a control-calculated risk of a bowed rotor can be computed as a bowed rotor risk parameter as further described herein and used to trigger a cockpit message or test stand message to extend a time period to run at idle power prior to engine shutdown as a pre-shutdown mitigation. Alternatively, the bowed rotor risk parameter can trigger a request message or automated initiation of a dry motoring sequence prior to engine start. During a dry motoring sequence, a starter air pressure valve can be modulated to limit high rotor speed below high spool resonance speed and prevent rub during dry motoring operation. The bowed rotor risk parameter can also be used to limit dry motoring duration to reduce the impact on air starter turbine life. The monitoring of vibration signatures during the entire engine starting sequence can also or separately be used to assess the risk of a bowed rotor start and direct maintenance, for instance, in the case of suspected outer air seal rub especially in the high compressor.

Rather than using sensed parameters of an engine system, embodiments use parameters at the aircraft level, e.g., as received on an aircraft communication bus, to derive parameters at the engine level, such as engine core temperature. For example, a profile of a throttle lever angle (TLA) parameter history can indicate a history of fuel flow requested over a period of time which may be used to infer an engine core temperature. By determining an amount of elapsed time from the last received valid set of aircraft parameters in combination with a known engine state, e.g., engine shutdown, a minimum dry motoring time can be calculated.

Referring now to FIG. 1, a schematic illustration of a gas turbine engine 10 is provided. The gas turbine engine 10 has among other components a fan through which ambient air is propelled into the engine housing, a compressor for pressurizing the air received from the fan and a combustor wherein the compressed air is mixed with fuel and ignited for generating combustion gases. The gas turbine engine 10 further comprises a turbine section for extracting energy from the combustion gases. Fuel is injected into the combustor of the gas turbine engine 10 for mixing with the compressed air from the compressor and ignition of the resultant mixture. The fan, compressor, combustor, and turbine are typically all concentric about a central longitudinal axis of the gas turbine engine 10. Thus, thermal deflection of the components of the gas turbine engine 10 may create the aforementioned bowing or “bowed rotor” condition along the common central longitudinal axis of the gas turbine engine 10 and thus it is desirable to clear or remove the bowed condition prior to the starting or restarting of the gas turbine engine 10.

FIG. 1 schematically illustrates a gas turbine engine 10 that can be used to power an aircraft, for example. The gas turbine engine 10 is disclosed herein as a multi-spool turbofan that generally incorporates a fan section 22, a compressor section 24, a combustor section 26 and a turbine section 28. The fan section 22 drives air along a bypass flowpath while the compressor section 24 drives air along a core flowpath for compression and communication into the combustor section 26 then expansion through the turbine section 28. Although depicted as a turbofan gas turbine engine in the disclosed non-limiting embodiment with two turbines and is sometimes referred to as a two spool engine, it should be understood that the concepts described herein are not limited to use with turbofans as the teachings may be applied to other types of turbine engines including three-spool architectures. In both of these architectures the starting spool is that spool that is located around the combustor, meaning the compressor part of the starting spool is flowing directly into the combustor and the combustor flows directly into the turbine section.

The engine 10 generally includes a low speed spool 30 and a high speed spool 32 mounted for rotation about an engine central longitudinal axis A relative to an engine static structure 36 via several bearing systems 38. It should be understood that various bearing systems 38 at various locations may alternatively or additionally be provided.

The low speed spool 30 generally includes an inner shaft 40 that interconnects a fan 42, a low pressure compressor 44 and a low pressure turbine 46. The inner shaft 40 is connected to the fan 42 through a geared architecture 48 to drive the fan 42 at a lower speed than the low speed spool 30 in the example of FIG. 1. The high speed spool 32 includes an outer shaft 50 that interconnects a high pressure compressor 52 and high pressure turbine 54. A combustor 56 is arranged between the high pressure compressor 52 and the high pressure turbine 54. A mid-turbine frame 57 of the engine static structure 36 is arranged generally between the high pressure turbine 54 and the low pressure turbine 46. The mid-turbine frame 57 further supports bearing systems 38 in the turbine section 28. The inner shaft 40 and the outer shaft 50 are concentric and rotate via bearing systems 38 about the engine central longitudinal axis A which is collinear with their longitudinal axes.

The core airflow is compressed by the low pressure compressor 44 then the high pressure compressor 52, mixed and burned with fuel in the combustor 56, then expanded over the high pressure turbine 54 and low pressure turbine 46. The mid-turbine frame 57 includes airfoils 59 which are in the core airflow path. The turbines 46, 54 rotationally drive the respective low speed spool 30 and high speed spool 32 in response to the expansion.

A number of stations for temperature and pressure are defined with respect to the gas turbine engine 10 according to conventional nomenclature. Station 2 is at an inlet of low pressure compressor 44 having a temperature T2 and a pressure P2. Station 2.5 is at an exit of the low pressure compressor 44 having a temperature T2.5 and a pressure P2.5. Station 3 is at an inlet of the combustor 56 having a temperature T3 and a pressure P3. Station 4 is at an exit of the combustor 56 having a temperature T4 and a pressure P4. Station 4.5 is at an exit of the high pressure turbine 54 having a temperature T4.5 and a pressure P4.5. Station 5 is at an exit of the low pressure turbine 46 having a temperature T5 and a pressure P5. Embodiments use aircraft-derived parameters rather than measured parameters at the stations depicted in FIG. 1 for bowed rotor mitigation.

Although FIG. 1 depicts one example configuration, it will be understood that embodiments as described herein can cover a wide range of configurations. For example, embodiments may be implemented in a configuration that is described as a “straddle-mounted” spool 32A of FIG. 9. This configuration places two bearing compartments 37A and 39A (which may include a ball bearing and a roller bearing respectively), outside of the plane of most of the compressor disks of high pressure compressor 52A and at outside at least one of the turbine disks of high pressure turbine 54A. In contrast with a straddle-mounted spool arrangement, other embodiments may be implemented using an over-hung mounted spool 32B as depicted in FIG. 10. In over-hung mounted spool 32B, a bearing compartment 37B is located forward of the first turbine disk of high pressure turbine 54B such that the high pressure turbine 54B is overhung, and it is physically located aft of its main supporting structure. The use of straddle-mounted spools has advantages and disadvantages in the design of a gas turbine, but one characteristic of the straddle-mounted design is that the span between the bearing compartments 37A and 39A is long, making the amplitude of the high spot of a bowed rotor greater and the resonance speed that cannot be transited prior to temperature homogenization is lower. For any thrust rating, the straddle mounted arrangement, such as straddle-mounted spool 32A, gives Lsupport/Dhpt values that are higher, and the overhung mounted arrangement, such as overhung spool 32B, can be as much as 60% of the straddle-mounted Lsupport/Dhpt. Lsupport is the distance between bearings (e.g., between bearing compartments 37A and 39A or between bearing compartments 37B and 39B), and Dhpt is the diameter of the last blade of the high pressure turbine (e.g., high pressure turbine 54A or high pressure turbine 54B). As one example, a straddle-mounted engine starting spool, such as straddle-mounted spool 32A, with a roller bearing at bearing compartment 39A located aft of the high pressure turbine 54A may be more vulnerable to bowed rotor problems since the Lsupport/Dhpt ranges from 1.9 to 5.6. FIGS. 9 and 10 also illustrate a starter 120 interfacing via a tower shaft 55 with the straddle-mounted spool 32A proximate high compressor 52A and interfacing via tower shaft 55 with the overhung mounted spool 32B proximate high compressor 52B as part of a starting system.

Turning now to FIG. 2, a schematic of a starting system 100 for the gas turbine engine 10 of FIG. 1 is depicted according to an embodiment. The starting system 100 is also referred to generally as a gas turbine engine system. In the example of FIG. 2, the starting system 100 includes a controller 102 which may be an electronic engine control, such as a dual-channel FADEC, and/or engine health monitoring unit. In an embodiment, the controller 102 may include memory to store instructions that are executed by one or more processors. The executable instructions may be stored or organized in any manner and at any level of abstraction, such as in connection with a controlling and/or monitoring operation of the engine 10 of FIG. 1. The one or more processors can be any type of central processing unit (CPU), including a general purpose processor, a digital signal processor (DSP), a microcontroller, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), or the like. Also, in embodiments, the memory may include random access memory (RAM), read only memory (ROM), or other electronic, optical, magnetic, or any other computer readable medium onto which is stored data and control algorithms in a non-transitory form.

The starting system 100 can also include a data storage unit (DSU) 104 that retains data between shutdowns of the gas turbine engine 10 of FIG. 1. The DSU 104 includes non-volatile memory and retains data between cycling of power to the controller 102 and DSU 104. An aircraft communication bus 106 can include an aircraft-level and/or test stand communication bus to interface with aircraft controls, e.g., a cockpit, various onboard computer systems, and/or a test stand. Based on detecting a bowed rotor start risk, the controller 102 can send a request for extended idle operation on the aircraft communication bus 106 prior to engine shutdown. Alternatively, the controller 102 can receive a request to initiate bowed rotor mitigation on the aircraft communication bus 106.

A motoring system 108 is operable to drive rotation of a starting spool (e.g., high speed spool 32) of the gas turbine engine 10 of FIG. 1. Either or both channels of controller 102 can alternate on and off commands to an electromechanical device 110 coupled to a discrete starter valve 116A to achieve a partially open position of the discrete starter valve 116A to control a flow from a starter air supply 114 (also referred to as air supply 114) through a transfer duct 118 to an air turbine starter 120 (also referred to as starter 120 or pneumatic starter motor 120) to drive rotation of a starting spool of the gas turbine engine 10 below an engine idle speed. The air supply 114 (also referred to as starter air supply 114) can be provided by any known source of compressed air, such as an auxiliary power unit or ground cart.

The controller 102 can monitor a speed sensor, such as speed pickup 122 that may sense the speed of the engine rotor through its connection to a gearbox 124 which is in turn connected to the high speed spool 32 via tower shaft 55 (e.g., rotational speed of high speed spool 32) or any other such sensor for detecting or determining the speed of the gas turbine engine 10 of FIG. 1. The starter 120 may be coupled to the gearbox 124 of the gas turbine engine 10 of FIG. 1 directly or through a transmission such as a clutch system (not depicted). The controller 102 can establish a control loop with respect to rotor speed to adjust positioning of the discrete starter valve 116A.

The discrete starter valve 116A is an embodiment of a starter valve that is designed as an on/off valve which is typically commanded to either fully opened or fully closed. However, there is a time lag to achieve the fully open position and the fully closed position. By selectively alternating an on-command time with an off-command time through the electromechanical device 110, intermediate positioning states (i.e., partially opened/closed) can be achieved. The controller 102 can modulate the on and off commands (e.g., as a duty cycle using pulse width modulation) to the electromechanical device 110 to further open the discrete starter valve 116A and increase a rotational speed of the starting spool of the gas turbine engine 10 of FIG. 1. In an embodiment, the electromechanical device 110 has a cycle time defined between an off-command to an on-command to the off-command that is at least half of a movement time for the discrete starter valve 116A to transition from fully closed to fully open. Pneumatic lines 112A and 112B or a mechanical linkage (not depicted) can be used to drive the discrete starter valve 116A between the open position and the closed position. The electromechanical device 110 can be a solenoid that positions the discrete starter valve 116A based on intermittently supplied electric power as commanded by the controller 102. In an alternate embodiment, the electromechanical device 110 is an electric valve controlling muscle air to adjust the position of the discrete starter valve 116A as commanded by the controller 102.

In the example of FIG. 2, the engine also includes a vibration monitoring system 126. The vibration monitoring system 126 includes at least one vibration pickup 128, e.g., an accelerometer, operable to monitor vibration of the gas turbine engine 10 of FIG. 1. Vibration signal processing 130 can be performed locally with respect to the vibration pickup 128, within the controller 102, or through a separate vibration processing system, which may be part of an engine health monitoring system to acquire vibration data 132. Alternatively, the vibration monitoring system 126 can be omitted in some embodiments.

Similar to FIG. 2, FIG. 3 is a schematic illustration of a starting system 100A for the gas turbine engine 10 of FIG. 1 in accordance with another embodiment. The starting system 100A includes controller 102 that controls motoring system 108A, as an alternate embodiment of the motoring system 108 of FIG. 2. Rather than using an electromechanical device 110 coupled to a discrete starter valve 116A to achieve a partially open position of the discrete starter valve 116A of FIG. 2, the motoring system 108A of FIG. 3 uses a variable position starter valve 116B. In FIG. 3, either or both channels of controller 102 can output a valve control signal 150 operable to dynamically adjust a valve angle of the variable position starter valve 116A that selectively allows a portion of the air supply 114 to pass through the variable position starter valve 116B and transfer duct 118 to air turbine starter 120. The variable position starter valve 116B is a continuous/infinitely adjustable valve that can hold a commanded valve angle, which may be expressed in terms of a percentage open/closed and/or an angular value (e.g., degrees or radians). Performance parameters of the variable position starter valve 116B can be selected to meet dynamic response requirements of the starting system 100A. For example, in some embodiments, the variable position starter valve 116B has a response rate of 0% to 100% open in less than 40 seconds. In other embodiments, the variable position starter valve 116B has a response rate of 0% to 100% open in less than 30 seconds. In further embodiments, the variable position starter valve 116B has a response rate of 0% to 100% open in less than 20 seconds.

The controller 102 can monitor a valve angle of the variable position starter valve 116B using valve angle feedback signals 152 provided to both channels of controller 102. As one example, in an active/standby configuration, both channels of the controller 102 can use the valve angle feedback signals 152 to track a current valve angle, while only one channel designated as an active channel outputs valve control signal 150. Upon a failure of the active channel, the standby channel of controller 102 can take over as the active channel to output valve control signal 150. In an alternate embodiment, both channels of controller 102 output all or a portion of a valve angle command simultaneously on the valve control signals 150. The controller 102 can establish an outer control loop with respect to rotor speed and an inner control loop with respect to the valve angle of the variable position starter valve 116B.

As in the example of FIG. 2, the starting system 100A of FIG. 3 also includes vibration monitoring system 126. The vibration monitoring system 126 includes at least one vibration pickup 128, e.g., an accelerometer, operable to monitor vibration of the gas turbine engine 10 of FIG. 1. Vibration signal processing 130 can be performed locally with respect to the vibration pickup 128, within the controller 102, or through a separate vibration processing system, which may be part of an engine health monitoring system to acquire vibration data 132. Alternatively, the vibration monitoring system 126 can be omitted in some embodiments.

FIG. 4 is a block diagram of a system 200 for bowed rotor start mitigation that may control the discrete starter valve 116A of FIG. 2 or the variable position starter valve 116B of FIG. 3 via control signals 210 in accordance with an embodiment. The system 200 may also be referred to as a bowed rotor start mitigation system. In the example of FIG. 3, the system 200 includes a mission risk factor model 204 which may be part of controller 102.

The mission risk factor model 204 determines at least one inferred engine operating thermal parameter, such as an engine temperature value or heat state (T_(core)) of the gas turbine engine 10, based on at least one aircraft-based parameter 202 received on the aircraft communication bus 106. The at least one aircraft-based parameter 202 is an aircraft-level parameter which is not directly observable by engine system sensors coupled to the controller 102. As one example, the at least one aircraft-based parameter 202 can be a throttle lever angle (TLA) profile indicate of a history of fuel demand for the aircraft. By observing the at least one aircraft-based parameter 202 over a period of time, the mission risk factor model can infer an engine operating thermal parameter, such as T_(core). For instance, T_(core) can be determined by relating flight profile information, e.g., altitude, thrust demand, time, etc., with known performance characteristics of the gas turbine engine 10 of FIG. 1 using a series of computations and/or look-up table(s) stored in memory of controller 102. The heat state of the engine 10 during use or T_(core) is inferred by the mission risk factor model 204 as the engine 10 is being run and valid instances of at least one aircraft-based parameter 202 are received on the aircraft communication bus 106. The mission risk factor model 204 can infer T_(core) absent sensed values at the engine level.

At engine shutdown, the current or most recently determined heat state of the engine or T_(core shutdown) of the engine 10 is recorded into DSU 104, and the time of the engine shutdown t_(shutdown) is recorded into the DSU 104. Time values and other parameters may be received on aircraft communication bus 106.

During an engine start sequence or restart sequence, a bowed rotor start risk model 206 of the controller 102 is provided with the data stored in the DSU 104, namely T_(core shutdown) and the time of the engine shutdown t_(shutdown). In addition, the bowed rotor start risk model 206 is also provided with the time of engine start t_(start). Although the mission risk factor model 204 and the bowed rotor start risk model 206 are separately depicted, it will be understood that the mission risk factor model 204 and the bowed rotor start risk model 206 may be combined as a single model.

The bowed rotor start risk model 206 maps core temperature model data inferred from the at least one aircraft-based parameter 202 with time data to establish a motoring time t_(motoring) as an estimated period of motoring to mitigate a bowed rotor of the gas turbine engine 10. The motoring time t_(motoring) is indicative of a bowed rotor risk parameter computed by the bowed rotor start risk model 206. The bowed rotor risk parameter may be quantified according to a profile curve 402 selected from a family of curves 404 that align with observed aircraft conditions, such as a longer duration of a higher TLA value, which increases turbine bore temperature and the resulting bowed rotor risk as depicted in the example graph 400 of FIG. 6. For instance, a higher risk of a bowed rotor may result in a longer duration of dry motoring to reduce a temperature gradient prior to starting the gas turbine engine 10 of FIG. 1. As will be discussed herein and in one embodiment, an engine start sequence may automatically include a modified start sequence; however, the duration of the modified start sequence prior to a normal start sequence will vary based upon the time period t_(motoring) that is calculated by the bowed rotor start risk model 206. The motoring time t_(motoring) for predetermined target speed N_(target) of the engine 10 can be calculated as a function of T_(core shutdown), t_(shutdown), and t_(start), (e.g., f (T_(core shutdown), t_(shutdown), and t_(start)), while a target speed N_(target) is a predetermined speed that can be fixed or vary within a predetermined speed range of N_(targetMin) to N_(targetMax). In other words, the target speed N_(target) can be the same regardless of the calculated time period t_(motoring) or may vary within the predetermined speed range of N_(targetMin) to N_(targetMax). The target speed N_(target) may also be referred to as a dry motoring mode speed.

Based upon these values (T_(core shutdown), t_(shutdown), and t_(start)) the motoring time t_(motoring) at a predetermined target speed N_(target) for the modified start sequence of the engine 10 is determined by the bowed rotor start risk model 206. Based upon the calculated time period t_(motoring) which is calculated as a time to run the engine 10 at a predetermined target speed N_(target) in order to clear a “bowed condition”. In accordance with an embodiment of the disclosure, the controller 102 can run through a modified start sequence upon a start command given to the engine 10 by an operator of the engine 10 such as a pilot of an airplane the engine is used with. It being understood that the motoring time t_(motoring) of the modified start sequence may be in a range of 0 seconds to minutes, which, of course, depends on the values of T_(core shutdown), t_(shutdown), and t_(start).

In an alternate embodiment, the modified start sequence may only be run when the bowed rotor start risk model 206 has determined that the motoring time t_(motoring) is greater than zero seconds upon receipt of a start command given to the engine 10. In this embodiment and if the bowed rotor start risk model 206 has determined that t_(motoring) is not greater than zero seconds, a normal start sequence will be initiated upon receipt of a start command to the engine 10.

Accordingly and during an engine command start, the bowed rotor start risk model 206 of the system 200 is automatically referenced wherein the bowed rotor start risk model 206 correlates the elapsed time since the last engine shutdown time and the shutdown heat state of the engine 10 as well as the current start time t_(start) in order to determine the duration of the modified start sequence wherein motoring of the engine 10 at a reduced speed N_(target) without fuel and ignition is required. As used herein, motoring of the engine 10 in a modified start sequence refers to the turning of a starting spool by the starter 120 at a reduced speed N_(target) without introduction of fuel and an ignition source in order to cool the engine 10 to a point wherein a normal start sequence can be implemented without starting the engine 10 in a bowed rotor state. In other words, cool or ambient air is drawn into the engine 10 while motoring the engine 10 at a reduced speed in order to clear the “bowed rotor” condition, which is referred to as a dry motoring mode.

The bowed rotor start risk model 206 can output the motoring time t_(motoring) to a motoring controller 208. The motoring controller 208 uses a dynamic control calculation in order to determine a required valve position of the starter valve 116A, 116B used to supply an air supply or starter air supply 114 to the engine 10 in order to limit the motoring speed of the engine 10 to the target speed N_(target) due to the position of the starter valve 116A, 116B. The required valve position of the starter valve 116A, 116B can be determined based upon an air supply pressure as well as other factors including but not limited to ambient air temperature of the aircraft, parasitic drag on the engine 10 from a variety of engine driven components such as electric generators and hydraulic pumps, and other variables such that the motoring controller 208 closes the loop for an engine motoring speed target N_(target) for the required amount of time based on the output of the bowed rotor start risk model 206. In one embodiment, the dynamic control of the valve position (e.g., open state of the valve (e.g., fully open, ½ open, ¼ open, etc.) in order to limit the motoring speed of the engine 10) is controlled via duty cycle control (on/off timing using pulse width modulation) of electromechanical device 110 for discrete starter valve 116A.

When the variable position starter valve 116B of FIG. 3 is used, a valve angle 207 can be provided to motoring control 208 based on the valve angle feedback 112 of FIG. 3. A rotor speed N2 (i.e., speed of high speed spool 32) can be provided to the motoring controller 208 and a mitigation monitor 214, where motoring controller 208 and a mitigation monitor 214 may be part of controller 102. Vibration data 132 can also be provided to mitigation monitor 214.

The risk model 206 can determine a bowed rotor risk parameter that is based on the heat stored (T_(core)) using a mapping function or lookup table. When not implemented as a fixed rotor speed, the bowed rotor risk parameter can have an associated dry motoring profile defining a target rotor speed profile over an anticipated amount of time for the motoring controller 208 to send control signals 210, such as valve control signals 150 for controlling variable position starter valve 116B of FIG. 3.

In some embodiments, an anticipated amount of dry motoring time can be used to determine a target rotor speed profile in a dry motoring profile for the currently observed conditions. As one example, one or more baseline characteristic curves for the target rotor speed profile can be defined in tables or according to functions that may be rescaled to align with the observed conditions. An example of a target rotor speed profile 1002 is depicted in graph 1000 of FIG. 12 that includes a steep initial transition portion 1004, followed by a gradually increasing portion 1006, and a late acceleration portion 1008 that increases rotor speed above a critical rotor speed, through a fuel-on speed and an engine idle speed. The target rotor speed profile 1002 can be rescaled with respect to time and/or select portions (e.g., portions 1004, 1006, 1008) of the target rotor speed profile 1002 can be individually or collectively rescaled (e.g., slope changes) with respect to time to extend or reduce the total motoring time. The target rotor speed profile 1002 may include all positive slope values such that the actual rotor speed 1010 is driven to essentially increase continuously while bowed rotor start mitigation is active. While the example of FIG. 12 depicts one example of the target rotor speed profile 1002 that can be defined in a dry motoring profile, it will be understood that many variations are possible in embodiments.

An example of the effects of bowed rotor mitigation are illustrated in graph 420 of FIG. 7 that depicts a normal or cooled engine start (line 432) versus a bowed rotor or mitigated engine start (line 434) in accordance with one non-limiting embodiment of the disclosure. At point 436, a pilot or operator of the engine 10 sets or initiates a start command of the engine. At point 438 and after the start command is initiated, the controller 102, based upon the risk model 206, requires the engine to motor at a pre-determined speed (N_(pre-determined) or N_(target)), which is less than a normal idle start speed N2 for a time (t_(determined)). The pre-determined speed (N_(pre-determined) or N_(target)) can be defined within a predetermined speed range N_(targetMin) to N_(targetMax) that is used regardless of the calculated time period t_(motoring) for homogenizing engine temperatures. The time period t_(determined) is based upon the output of the risk model 206. The determined speed (N_(pre-determined) or N_(target)) is achieved by controlling the operational position of starter valve 116A, 116B. Thereafter and at point 440 when the required motoring time (determined from the risk model 206) has been achieved, such that the “bowed condition” has been cleared a normal start sequence with a normal speed N2 is initiated. Subsequently and at point 442, the idle speed N2 has been achieved. This modified sequence is illustrated in one non-limiting manner by the dashed line 434 of the graph 420 of FIG. 7. It is, of course, understood that (t_(determined)) may vary depending upon the outputs of the risk model 206, while N_(pre-determined) or N_(target) is a known value. Of course, in alternative embodiments, the risk model 206 may be configured to provide the speed of the engine 10 during a modified start sequence. Still further and as mentioned above, the starter valve may be dynamically varied based upon the outputs of the risk model 206 as well as the pressure of the air supply 114 in order to limit the motoring speed of the engine 10 to that of N_(pre-determined) or N_(target) during the clearing of a bowed rotor condition. Line 432 illustrates a normal start sequence wherein the time t_(determined) is zero for a modified start as determined by the risk model 206.

The example of FIG. 11 illustrates how a valve angle command 902 can be adjusted between 0 to 100% of a commanded starter valve opening to generate the actual rotor speed 1010 of FIG. 12. As the actual rotor speed 1010 tracks to the steep initial transition portion 1004 of the target rotor speed profile 1002, the valve angle command 902 transitions through points “a” and “b” to fully open the variable position starter valve 116B. As the slope of the target rotor speed profile 1002 is reduced in the gradually increasing portion 1006, the valve angle command 902 is reduced between points “b” and “c” to prevent the actual rotor speed 1010 from overshooting the target rotor speed profile 1002. In some embodiments, decisions to increase or decrease the commanded starter valve opening is based on monitoring a rate of change of the actual rotor speed 1010 and projecting whether the actual rotor speed 1010 will align with the target rotor speed profile 1002 at a future time. If it is determined that the actual rotor speed 1010 will not align with the target rotor speed profile 1002 at a future time, then the valve angle of the variable position starter valve 116B is adjusted (e.g., increase or decrease the valve angle command 902) at a corresponding time. In the example of FIGS. 11 and 12, the valve angle command 902 oscillates with a gradually reduced amplitude between points “c”, “d”, and “e” as the actual rotor speed 1010 tracks to the target rotor speed profile 1002 through the gradually increasing portion 1006. As dry motoring continues, the overall homogenization of the engine 10 increases, which allows the actual rotor speed 1010 to safely approach the critical rotor speed without risking damage. The valve angle command transitions from point “e” to point “f” and beyond to further increase the actual rotor speed 1010 in the late acceleration portion 1008 above the critical rotor speed, through a fuel-on speed and an engine idle speed. By continuously increasing the actual rotor speed 1010 during dry motoring, the bowed rotor condition can be reduced faster than holding a constant slower speed.

As one example of an aircraft that includes systems as described herein, mission risk factor model 204 may run on controller 102 of the aircraft to track heat stored (T_(core)) in the turbine at the time of engine shutdown. Modeling of potential heat stored in the system may be performed as a turbine disk metal temperature model in the mission risk factor model 204. When the aircraft lands, engines typically operate at idle for a cool down period of time, e.g., while taxiing to a final destination. When an engine shutdown is detected, model state data can be logged by the DSU 104 prior to depowering. When the controller 102 powers on at a later time and model state data can be retrieved from the DSU 104, and the bowed rotor start risk model 206 can be updated to account for the elapsed time. When an engine start is requested, a bowed rotor risk can be assessed with respect to the bowed rotor start risk model 206. Extended dry motoring can be performed during an engine start process until the bow risk has sufficiently diminished. If a safety condition is detected by the controller 102, for instance, a notification that maintenance will be performed, an abort signal 220 can be triggered to halt dry motoring.

In reference to FIGS. 4 and 12, the mitigation monitor 214 of FIG. 4 can operate in response to receiving a complete indicator 212 to run a verification of the bowed rotor mitigation. The mitigation monitor 214 can provide mitigation results 216 to the motoring controller 208 and may provide result metrics 218 to other systems, such a maintenance request or indicator. Peak vibrations can be checked by the mitigation monitor 214 during the start processes to confirm that bowed rotor mitigation successfully removed the bowed rotor condition. The mitigation monitor 214 may also run while dry motoring is active to determine whether adjustments to the dry motoring profile are needed. For example, if a greater amount of vibration is detected than was expected, the mitigation monitor 214 can request that the motoring controller 208 reduce a slope of the target rotor speed profile 1002 of FIG. 12 to extend the dry motoring time before driving the actual rotor speed 1010 of FIG. 12 up to the critical rotor speed. Similarly, if the magnitude of vibration observed by the mitigation monitor 214 is less than expected, the mitigation monitor 214 can request that the motoring controller 208 increase a slope of the target rotor speed profile 1002 of FIG. 12 to reduce the dry motoring time before driving the actual rotor speed 1010 of FIG. 12 up to the critical rotor speed.

FIG. 5 is a flow chart illustrating a method 300 of bowed rotor start mitigation of a gas turbine engine in accordance with an embodiment. The method 300 of FIG. 5 is described in reference to FIGS. 1-12 and may be performed with an alternate order and include additional steps. At block 302, the controller 102 receives at least one aircraft-based parameter 202 on aircraft communication bus 106. At block 304, the controller 102 determines at least one inferred engine operating thermal parameter (e.g., T_(core)) from the at least one aircraft-based parameter 202 (e.g., TLA). The at least one inferred engine operating thermal parameter can be selected from data describing a history of the aircraft before an engine shutdown.

At block 306, the controller 102 controls the motoring system 108, 108A to drive rotation of a starting spool of the gas turbine engine 10 below an engine idle speed based on determining that the at least one inferred engine operating thermal parameter is within a preselected threshold. The preselected threshold may be a value or range where a greater risk of a bowed rotor condition exists. The motoring system 108, 108A can be controlled as described above depending on the system implementation. The controller 102 may transmit a notification on the aircraft communication bus 106 that the gas turbine engine 10 is being conditioned for starting to inform aircraft crew and/or a test stand operator. The time since the engine shutdown t_(shutdown) can be used to modify a time of rotation for dry motoring. If time data is unavailable from the DSU 104, dry motoring can be performed for a maximum expected time assuming worst case conditions.

If the controller 102 detects a safety condition (which may include data received on the aircraft communication bus 106) while dry motoring is active, the controller 102 can trigger abort signal 220 to abort rotation of the starting spool. A braking torque may be applied to the starting spool to abort rotation of the starting spool based on the abort signal 220. The controller 102 can also monitor for rotary motion of the shaft using a rotation sensor, such as speed pickup 122 or an alternate sensor that detects rotor speed N2. The controller 102 can trigger removal of power from the motoring system 108, 108A based on detecting rotary motion of the starting spool above a target speed (e.g., target speed N_(target)). For example, dry motoring may not be needed if wind or other forces drive rotation of the high speed spool 32.

In embodiments, the risk model 206 can be used to determine a motoring time period t_(motoring) for a start sequence of the gas turbine engine 10, where the risk model 206 uses the recorded time of the engine shutdown and the stored heat state of the gas turbine engine 10 at shut down to determine the motoring time period t_(motoring). The gas turbine engine 10 is motored at a predetermined speed range of N_(targetMin) to N_(targetMax) during the motoring time period, which is less than a normal idle start speed N2. The controller 102 can dynamically vary a position of starter valve 116A, 116B during the motoring time period in order to motor the gas turbine engine 10 at the predetermined speed range of N_(targetMin) to N_(targetMax). The predetermined speed range of N_(targetMin) to N_(targetMax) may be tightly controlled to a substantially constant rotor speed or cover a wider operating range according to a dry motoring profile.

As one example with respect to FIGS. 3 and 12, the variable position starter valve 116B can be initially set to a valve angle of greater than 50% open when bowed rotor start mitigation is active. The controller 102 can monitor a rate of change of the actual rotor speed 1010, project whether the actual rotor speed 1010 will align with the target rotor speed profile 1002 at a future time based on the rate of change of the actual rotor speed 1010, and adjust a valve angle of the variable position starter valve 116B based on determining that the actual rotor speed 1010 will not align with the target rotor speed profile 1002 at a future time.

Further dynamic updates at runtime can include adjusting a slope of the target rotor speed profile 1002 in the dry motoring profile while the bowed rotor start mitigation is active based on determining that a vibration level of the gas turbine engine 10 is outside of an expected range. Adjusting the slope of the target rotor speed profile 1002 can include maintaining a positive slope. Vibration levels may also or alternatively be used to check/confirm successful completion of bowed rotor start mitigation prior to starting the gas turbine engine 10. For instance, based on determining that the bowed rotor start mitigation is complete, a vibration level of the gas turbine engine 10 can be monitored while sweeping through a range of rotor speeds including the critical rotor speed.

In further reference to FIG. 4, the mitigation monitor 214 of FIG. 4 may receive a complete indicator 212 from the motoring controller 208 when the motoring controller 208 has completed dry motoring, for instance, if the motoring time has elapsed. If the mitigation monitor 214 determines that the bowed rotor condition still exists based on vibration data 132 collected, the motoring controller 208 may restart dry motoring, or a maintenance request or indicator can be triggered along with providing result metrics 218 for further analysis. Metrics of attempted bowed rotor mitigation can be recorded in the DSU 104 based on determining that the attempted bowed rotor mitigation was unsuccessful or incomplete.

Referring now to FIG. 8, a graph 500 illustrating examples of various vibration level profiles 502 of an engine, such as gas turbine engine 10 of FIG. 1 is depicted. The vibration level profiles 502 represent a variety of possible vibration levels observed before and/or after performing bowed rotor mitigation. Critical rotor speed 510 is the speed at which a vibration peak is expected due to amplification effects of a bowed rotor condition along with other contributions to vibration level generally. A peak vibration 504 at critical rotor speed 510 may be used to trigger different events. For example, if the peak vibration 504 at critical rotor speed 510 is below a maintenance action threshold 506, then no further actions may be needed. If the peak vibration 504 at critical rotor speed 510 is above a damage risk threshold 508, then an urgent maintenance action may be requested such as an engine check. If the peak vibration 504 at critical rotor speed 510 is between the maintenance action threshold 506 and the damage risk threshold 508, then further bowed rotor mitigation actions may be requested, such as extending/restarting dry motoring. In one embodiment, a maintenance request is triggered based on the actual vibration level exceeding maintenance action threshold 506 after completing an attempt of bowed rotor mitigation.

The lowest rotor vibration vs. speed in FIG. 8 (vibration profile 502D) is for a fully homogenized rotor, where mitigation is not necessary (engine parked all night long, for example). The next higher curve shows a mildly bowed rotor and so on. The maintenance action threshold 506 is a threshold for setting a maintenance flag such as requiring a troubleshooting routine of one or more system elements. The damage risk threshold 508 may be a threshold to trigger a more urgent maintenance requirement up to and including an engine check. As dry motoring is performed in embodiments, the gas turbine engine 10 may shift between vibration profiles. For instance, when a bow rotor condition is present, the gas turbine engine 10 may experience vibration levels according to vibration profile 502A, if mitigation is not performed. As dry motoring is run, the gas turbine engine 10 may have a vibration profile that is gradually reduced from vibration profile 502A to vibration profile 502B and then vibration profile 502C, for example. By checking the current vibration level at a corresponding rotor speed with respect to time, the controller 102 can determine whether adjustments are needed to extend or reduce the slope of the target rotor speed profile 1002 of FIG. 12 depending on an expected rate of bowed rotor reduction. In embodiments, a slope of the target rotor speed profile 1002 in the dry motoring profile 206 can be adjusted and maintains a positive slope while bowed rotor start mitigation is active based on determining that a vibration level of the gas turbine engine 10 is less than a targeted maximum range 512, which may define a safe level of vibration to ensure that no risk of a maintenance action or damage will likely occur if the actual rotor speed 1010 is increased faster than previously planned.

Accordingly and as mentioned above, it is desirable to detect, prevent and/or clear a “bowed rotor” condition in a gas turbine engine that may occur after the engine has been shut down. As described herein and in one non-limiting embodiment, the controller 102 may be programmed to automatically take the necessary measures in order to provide for a modified start sequence without pilot intervention other than the initial start request. In an exemplary embodiment, the controller 102 and/or DSU 104 comprises a microprocessor, microcontroller or other equivalent processing device capable of executing commands of computer readable data or program for executing a control algorithm and/or algorithms that control the start sequence of the gas turbine engine. In order to perform the prescribed functions and desired processing, as well as the computations therefore (e.g., the execution of Fourier analysis algorithm(s), the control processes prescribed herein, and the like), the controller 102 and/or DSU 104 may include, but not be limited to, a processor(s), computer(s), memory, storage, register(s), timing, interrupt(s), communication interfaces, and input/output signal interfaces, as well as combinations comprising at least one of the foregoing. For example, the controller 102 and/or DSU 104 may include input signal filtering to enable accurate sampling and conversion or acquisitions of such signals from communications interfaces. As described above, exemplary embodiments of the disclosure can be implemented through computer-implemented processes and apparatuses for practicing those processes.

While the present disclosure has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the present disclosure is not limited to such disclosed embodiments. Rather, the present disclosure can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the present disclosure. Additionally, while various embodiments of the present disclosure have been described, it is to be understood that aspects of the present disclosure may include only some of the described embodiments. Accordingly, the present disclosure is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims. 

1. A bowed rotor start mitigation system for a gas turbine engine of an aircraft, the bowed rotor start mitigation system comprising: a motoring system; and a controller coupled to the motoring system and an aircraft communication bus, wherein the controller is configured to perform: determining at least one inferred engine operating thermal parameter from at least one aircraft-based parameter received on the aircraft communication bus; and controlling the motoring system to drive rotation of a starting spool of the gas turbine engine below an engine idle speed based on determining that the at least one inferred engine operating thermal parameter is within a preselected threshold.
 2. The bowed rotor start mitigation system as in claim 1, wherein the at least one inferred engine operating thermal parameter is selected from data describing a history of the aircraft before an engine shutdown.
 3. The bowed rotor start mitigation system as in claim 2, wherein a time since the engine shutdown is used to modify a time of rotation of the starting spool by the motoring system.
 4. The bowed rotor start mitigation system as in claim 1, wherein the controller transmits a notification on the aircraft communication bus that the gas turbine engine is being conditioned for starting.
 5. The bowed rotor start mitigation system as in claim 1, wherein the controller is configured to abort rotation by the motoring system when a safety condition is detected.
 6. The bowed rotor start mitigation system as in claim 5, wherein the motoring system is configured to apply a braking torque to the starting spool to abort rotation of the starting spool.
 7. The bowed rotor start mitigation system as in claim 1, further comprising a rotation sensor for detecting rotary motion of the starting spool, and the controller is further coupled to the rotation sensor.
 8. The bowed rotor start mitigation system as in claim 7, wherein the controller is configured to remove power from the motoring system based on detecting rotary motion of the starting spool above a target speed.
 9. The bowed rotor start mitigation system as in claim 1, wherein a vibration monitor is used to set a maintenance flag.
 10. The bowed rotor start mitigation system as in claim 9, wherein the at least one inferred engine operating thermal parameter comprises an engine temperature value, and the at least one aircraft-based parameter received on the aircraft communication bus comprises a throttle lever angle profile.
 11. A method of bowed rotor start mitigation for a gas turbine engine of an aircraft, the method comprising: determining at least one inferred engine operating thermal parameter from at least one aircraft-based parameter received on an aircraft communication bus; and controlling a motoring system to drive rotation of a starting spool of the gas turbine engine below an engine idle speed based on determining that the at least one inferred engine operating thermal parameter is within a preselected threshold.
 12. The method as in claim 11, wherein the at least one inferred engine operating thermal parameter is selected from data describing a history of the aircraft before an engine shutdown.
 13. The method as in claim 12, wherein a time since the engine shutdown is used to modify a time of rotation of the starting spool by the motoring system.
 14. The method as in claim 11, further comprising: transmitting a notification on the aircraft communication bus that the gas turbine engine is being conditioned for starting.
 15. The method as in claim 11, further comprising: aborting rotation by the motoring system when a safety condition is detected.
 16. The method as in claim 15, further comprising: applying a braking torque to the starting spool to abort rotation of the starting spool.
 17. The method as in claim 11, further comprising: detecting rotary motion of the starting spool using a rotation sensor.
 18. The method as in claim 17, further comprising: removing power from the motoring system based on detecting rotary motion of the starting spool above a target speed.
 19. The method as in claim 11, further comprising: setting a maintenance flag based on a vibration monitor.
 20. The method as in claim 19, wherein the at least one inferred engine operating thermal parameter comprises an engine temperature value, and the at least one aircraft-based parameter received on the aircraft communication bus comprises a throttle lever angle profile. 